1. Field of the Invention
The present invention relates to an apparatus for transmitting a synchronization message used to synchronize a communication network.
The apparatus is employed in, for example, a Synchronous Optical Network (SONET) that conforms to a new international standard for optical transmission. SONET was formulated by the Exchange Carriers Standards Association for the American National Standards Institute (ANSI). SONET has been adopted by the Consultive Committee for International Telephone and Telegraph (CCITT).
The SONET interface standard defines the physical interface, optical line rates known as Optical Carrier (OC) signals, frame format, and an Operations, Administration, Maintenance and Provisioning (OAM&P) protocol. SONET allows many advanced networking capabilities and, at the same time, provides transport for existing services.
Optical Carrier (OC) signals have electrical equivalents called Synchronous Transport Signals (STSs). The base rate is 51.84 Mb/s (OC-1/STS-), and higher rates are multiples of the base rate. For example, OC-12=12 .times.51.84 Mb/s, or 622.080 Mb/s. Higher rate SONET signals are produced by byte-interleaving N STS-1s (N=any whole number) to form an STS-N signal. The STS-N is then converted to an Optical Carrier-Level N (OC-N) signal. The optical OC-N signal is formed by scrambling the electrical STS-N signal using a present polynomial, which was selected to improve laser longevity. The OC-N has a line rate of exactly N times the OC-1 signal (51.84 Mb/s).
SONET transmission equipment interleaves STSs to form a synchronous high-speed signal. This method permits access to low-speed signals, e.g., DS0 (digital signal 0), DS1 (digital signal 1), etc., without multistage multiplexing and demultiplexing.
SONET stipulates a synchronization message that specifies the quality level or accuracy of a clock used to synchronize the network. It is necessary to provide a function of transmitting the synchronization message with the overhead of a SONET signal or with the data link bits of ESF (extended superframe format) frames consisting of DS1, as well as a function of selecting a master clock of the best quality according to the synchronization message.
2. Description of the Related Art
Although SONET stipulates the transmission and reception of a synchronization message and the switching of a reference clock under a normal state, it insufficiently stipulates how to deal with errors. For example, a synchronization message inserting unit, a signal transmitting unit, a system clock or reference clock selecting unit, etc., incorporated in a synchronization message transmitting apparatus may cause an unequipped error, an erroneously mounted error, or an internal fault. It is also possible that manual switches are erroneously operated to select, as a reference clock, an external clock whose quality is lower than an internal clock. If such an error or mistake occurs, the actual quality of a reference clock used to transmit a frame to an opposite party will be lower than the clock quality written in a synchronization message enclosed in the frame. SONET insufficiently stipulates how to deal with this kind of trouble.